How the Flu Virus Changes

Flu viruses constantly change and mutate. Sometimes these mutations result in viruses that move from animals to humans.

Antigenic drift refers to changes to the flu virus that happen slowly over time. This causes the changes to the seasonal flu that require us to get vaccinated against the flu each year.

Antigenic shift results when two different flu strains combine and infect the same cell. This mutation is what allows flu viruses to move from animals to humans.

How do flu viruses change?

Flu viruses are constantly changing and mutating. These changes can happen slowly over time or suddenly.

Antigenic drift is when these changes happen slowly over time. These changes happen often enough that your immune system can’t recognize the flu virus from year to year. That is why you need to get a new flu vaccine each year. The flu vaccine protects you against that season’s three or four most common flu virus strains.

Antigenic shift is when changes happen suddenly. This occurs when two different flu strains infect the same cell and combine. This may create a new flu subtype. Because people have little or no immunity to the new subtype, it can cause a very severe flu epidemic or pandemic.

How do flu viruses move from animals to humans?

Flu viruses move from animals to humans because of antigenic shift. Antigenic shift can happen in three ways:

Antigenic Shift 1

A duck or other aquatic bird passes a bird flu strain to an intermediate host such as a chicken or pig.

A person passes a human strain of influenza A to the same chicken or pig.

When the viruses infect the same cell, the genes from the bird strain mix with genes from the human strain, making a new strain.

The new strain can spread from the intermediate host to humans.

Antigenic Shift 2

A bird strain of flu can jump directly from a duck or other aquatic bird to humans.

Antigenic Shift 3

A bird strain of flu can jump directly from a duck or other aquatic bird to an intermediate animal host, such as a chicken or pig, and then to humans.

The new strain may evolve and spread rapidly from person to person. If so, it could result in a pandemic.